The existence of hot spots along part of the high voltage power cable route must generally be taken into account in high voltage power cable design. Hot spots reduce the ampacity, i.e. the maximum amount of electrical current a conductor or device can carry before sustaining immediate or progressive deterioration, of a power cable. More commonly than conductor deterioration as such, heat generated in the conductor may damage the electrical insulating system that insulates the conductor.
Hot spots occur due to environmental influences in the proximity of the power cable. This may for example be a result of the specific composition of the soil along the power cable route, which in some areas may have insufficient heat dissipating properties, resulting in higher ambient temperature for a heat-emitting power cable. Another example is a location where the power cable route passes external heat sources, for example when several power cables are closely located.
Due to hot spots, historically, an entire high voltage cable would be designed according to the worst conditions that occur along the high voltage cable route. This would normally mean that the diameter of the entire cable had to be dimensioned based on the worst conditions, resulting in over-dimensioning of the cable, and high costs associated therewith.
A known solution to the above problem is to adapt the conductor of a high voltage power cable along the power cable route, based on the conditions along this route. The same power transfer capacity may thereby be achieved along the entire length of the cable. For this purpose, the conductive core of a high voltage power cable may comprise several conductor sections having different cross-sectional layout or geometry. The majority of the length of the conductor may for example be of compacted type, which is a relatively inexpensive conductor configuration but which has a relatively low ampacity, rendering it more sensitive to ambient heat fluctuations. Sections of the conductor that are located in hot spots may be of a type that has a higher ampacity, and which therefore generally is more expensive. An example of such a conductor is one that is of segmented type, i.e. a Milliken conductor.
Jointing of different conductor sections, e.g. a conductor of compacted type and one of segmented type, as described above normally involves a bolt connection of the conductors, wherein a joint body encloses the two jointed conductors for each electrical phase. An external sleeve or collar encloses the joint bodies of all the electrical phases of the jointed power cable thus forming a stiff or rigid joint. In case the power cable has armour wires, these may be clamped or welded to the external sleeve.